v0.4.1: HBase Property Pushdown added

• Cypher, Subgraph and SNA Benchmarks added
• Predicate pushdown for HBase available
• Split property type and value in HBase table layout
• NULL and Set support in PropertyValues
• Code coverage report
• Support graphs without edges
• .print() added for LogicalGraph and GraphCollection
• Aggregation example added
• Sampling algorithms extended incl. Graph density computation